1. Field of the Invention:
The present invention relates to an amplifying circuit for push-pull amplifying an input signal and more particularly to a push-pull power amplifying circuit of such a type that crossover distortion is compensated for by supplying an idling current through a current mirror circuit while reducing the current consumption.
2. Description of the Related Art:
Heretofore, the output stage of audio instruments such as radios, tape recorders and so on has amplified input signals through such a push-pull amplifying circuit as shown in FIG. 2.
One such push-pull amplifying circuit is disclosed in Japanese Patent Laid-Open No. Sho 59-19415. As is well-known in the art, B class push-pull power amplifying circuits utilizing transistors are advantageous in that the power consumption at its output can be decreased. However, such a push-pull power amplifying circuit raises a problem in that the on-off operation of a complementary output transistor defining the output of the circuit causes crossover distortion in the output signal.
In the prior art system, such crossover distortion has been removed by using two current mirror circuits so that a small constant current passes through the output transistor even in a no signal state. Such a small constant current is known as idling current".
Referring now to FIG. 2, there is shown an amplifying circuit 1 which mainly comprises a first drive stage A1, a second drive stage A2, a first output stage including a current mirror circuit which is adapted to supply the idling current and includes transistors Tr7 and Tr11, and a second output stage including another current mirror circuit which is similarly adapted to supply the idling current and includes transistors Tr9 and Tr12.
It is assumed herein that the ratio of the emitter areas of the transistors Tr7 and Tr11 to the emitter areas of the transistors Tr9 and Tr12 is 1:N (where N is a positive real number).
An input signal A is applied to the first drive stage A1. The output current of the first drive stage A1 is supplied to the collector and base of the transistor Tr7. The transistors Tr7 and Tr11 are connected to each other in a current mirror relationship. Since the ratio of current mirroring is set to be 1:N, the collector current of the transistor Tr11 will be N.times.I1 if the output current of the first drive stage A1 is I1 (=I1DC+I1AC where I1DC is direct current and I1AC is alternating current). This collector current is then supplied to a capacitor C1 and a resistor R5 which is a load such as speaker or the like.
On the other hand, another input signal A having a phase opposite to that of the input signal A is applied to the second drive stage A2. The output current of the second drive stage A2 is supplied to the collector and base of the transistor Tr9. The transistors Tr9 and Tr12 are connected to each other in a current mirror relationship. Since the ratio of current mirroring is set to be 1:N, the collector current of the transistor Tr12 will be N.times.I2 if the output current of the second drive stage A2 is I2 (=I2DC+I2AC where I2DC is direct current and I2AC is alternating current). This collector current is then supplied to the resistor R5 through the capacitor C1 which serves as an a.c. coupler.
Since the input signals A and A are opposite to each other in phase, the output currents of the transistors Tr11 and Tr12 will be supplied to the load resistor R5 through the coupling capacitor C1 in a push-pull relationship between these output currents.
By utilizing these current mirror circuits, a constant direct current will pass through the outputs of the transistors Tr11 and Tr12. This direct current functions as an idling current to compensate for the crossover distortion, even if there are no input signals A and A.
Thus, the prior art amplifying circuit 1 must have an increased current mirror ratio (1:N) to improve the amplification factor thereof since the output amplification stages are connected with each other in the current mirror relationship. With a head-phone type stereo tape recorder, for example, it is preferred that the mirror ratio is about 1:120.
If the mirror ratio is increased, however, the idling current is also increased in a no-signal state. This results in an increase in current consumption.
If such a push-pull power amplifying circuit is to be constructed by the use of LSI, the mirror ratio should be defined by the ratio of emitter area in the transistors. This means that the emitters of these output transistors occupy an increased space, leading to an obstruction against the miniaturization of LSI.
It is therefore an object of the present invention to provide an improved amplifying circuit which can supply sufficient drive power to a load without an increase in the idling current.